Obesity is the most important predictor for obstructive sleep apnea. Although we know that obesity is assoc- iated with a decrease in pharyngeal airway size and an increase in pharyngeal airway collapsibility, we have almost no understanding of how obesity alters the mechanical properties of pharyngeal wall tissues to effect these changes. The PI has developed a novel application of MRI with tissue motion tracking (spatial modul- ation of magnetization, or SPAMM[unreadable]) to examine the relationships between regional changes in pharyngeal airway size and pharyngeal wall tissue motion in anesthetized rats. Using this innovative technique, we will determine the effects of obesity on pharyngeal mechanics by studying three groups of Zucker rats with different degrees of body fat: lean, obese, and even more obese rats fed a high-fat diet. Our global hypo- thesis is that obesity compromises pharyngeal airway patency through airway narrowing, increased airway collapsibility, and less effective muscle mechanics. Aim 1 is to determine 1) whether pharyngeal airway compliance and closing pressure progressively increase across age-matched Zucker rats in the order: lean <obese <fat-augmented obese rats, and 2) whether these findings are directly related to the amount of pharyngeal fat. Aim 2 is to determine changes in pharyngeal wall tissue motion for increasingly negative airway pressure up to induced airway collapse. MRI with SPAMM will be performed during application of subatmospheric pressure to the isolated, sealed upper aiway to test 1) whether airway collapse is primarily due to displacement of the anterior pharyngeal wall into the airway, and 2) whether, for a given pharyngeal region, the relationship between pharyngeal wall tissue motion and airway pressure decrements is progress- ively increased across age-matched Zucker rats in the order: lean <obese <fat-augmented obese rats. Aim 3 is to determine pharyngeal wall tissue motion in the 3 groups of Zucker rats during electrical activation of tongue protrudor and retractor muscles. The results will determine whether the relationship between wall tissue displacement and increase in airway size over a range of stimulus intensities is progressively reduced across age-matched Zucker rats in the order: lean >obese >fat augmented obese. The proposed studies will provide new information on how obesity affects pharyngeal mechanics and increase our understanding of why obese patients are predisposed to pharyngeal airway collapse during sleep.